Aerosols and the Residual Clear-Sky Insolation Discrepancy

The “clear-sky insolation discrepancy” surfaced a few years ago: several well-regarded theoretical simulations (sound radiative transfer codes and carefully measured inputs for them) produced values for clear-sky shortwave (SW) insolation that exceeded measurements from 20 to 30 Wm. Now, by both carefully screening (Long-Ackerman) the radiometer observations and including the record of the newly installed Eppley Black and White (B&W) pyranometer, we find theory exceeding observations by means of -2.1 Wm (total), -7.3 Wm (direct horizontal), and 5.2 Wm (diffuse) for 500 half-hourly observations during January to December 2000 at the Southern Great Plains (SGP) Central Facility (CF) C01 site. For moderate values of aerosol optical thickness (AOT), the aerosol forcing-to-surface insolation is considerably greater than the (now reduced) discrepancy of theory and observations. From a detailed look at the time series, the perspective is less rosy. The fine agreement in time mean for the direct horizontal (the component of flux, which can be most confidently measured) is produced by compensation: theory exceeds measurement for one period, and measurement exceeds theory for another. Results with permutations of Cimel versus multifilter rotating shadowband radiometer (MFRSR) for AOT, the use of different broadband instruments, and confinement to periods of agreement between duplicate measurements tell a similar story. These results cannot be satisfactorily explained as due to minor H2O effects that were not in the present simulation. With the current generation of observations, we approach a limit for matching them with simulations of the direct beam in an extended time series. This limit suggests that adjustments, for example, of soot fraction (here assumed 10 percent with a modified Fu-Liou code) to routinely assess aerosol absorption via comparison with the diffuse beam face the same barrier. The accurate assessment of anthropogenic forcing to the absorption of shortwave (SW) by the atmosphere yet remains beyond the grasp of climate science. At top-of-the-atmosphere (TOA) for reflected SW flux, simulations using surface albedos observed at the C01 site exceed the Terra (satellite) Clouds and the Earth’s Radiant Energy System (CERES) Earth Radiation Budget Experiment (ERBE)-like ES8 (ERBE Science) archival product by a mean of 27.0 Wm for a set of 44 footprints during 2000, which were carefully screened as cloud free. Eleventh ARM Science Team Meeting Proceedings, Atlanta, Georgia, March 19-23, 2001 2 When the identical footprints were compared with calculations based on surface albedos measured by radiometers at the adjacent E13 site, the broadband reflected from computations then exceeded CERES by 13.2 Wm.